1. Field of the Invention
This invention relates in general to an offset-removing gain circuit and method, and more particularly to a device and method which allow an offset to be independent of the forward path, and furthermore allows a high pass filter time constant to be independent of the forward path gain.
2. Description of Related Art
In general, every radio receiver is used for receiving an input signal in one form and for outputting the signal in another form. As a part in doing this, many radio receivers convert a received input signal to a baseband signal. The baseband signal is the original configuration of the signal, that is, without the intermediate modulation that may have been performed in order to facilitate transmission of the signal.
One example of a radio receiver is the kind used in a wireless voice communication system. Generally, this radio receiver may be located in a cellular telephone, allowing the cellular phones in the network to communicate, and also allowing communication between a telephone in the network and a telephone in another communication network. Typically, such a radio receiver will divide the received input signal into Inphase and Quadrature signals, prior to carrying out the demodulation of each signal into the respective baseband signal. Thus, a radio receiver may simultaneously be processing an Inphase baseband signal and a Quadrature baseband signal.
Baseband signals are often offset by unwanted direct current (DC) signals as they propagate through the Inphase or Quadrature baseband channels. While passing through the baseband channels, the baseband signal goes through a series of stages, and it is often desirable or necessary to remove the DC offset signal from the baseband signal, as the DC offset will also be amplified.
The order of magnitude of the received input signals may for example be about 100.mu. volts. Taking into consideration that the offset signal may be of the order of tens of millivolts, it is clear that such offsets can completely mask the desired signal if the offsets are allowed to accumulate.
There are systems and methods for removing the DC offset from the baseband signal. However, such systems and methods are associated with one or more disadvantages. One undesirable feature of prior art systems is that when the forward path gain in the circuit is altered during use, the offset-removing circuitry will experience a step response due to the gain change. The DC voltage output produced as a result of the step response may be misinterpreted as signal by subsequent components and circuitry.
Another disadvantage is that the high pass filter time constant associated with the offset-removing circuitry is often dependent on the forward path gain. As mentioned above, this leads to unwanted effects when altering the forward gain level during operation.
It can be seen, then, that there is a need for effective methods and techniques to remove DC offset in the baseband signal of a radio receiver, while allowing the detected offset to be independent of the forward path gain.
It can be seen that there is a need for a system for DC offset removal which is highly effective in removing DC offset while allowing the detected offset and the high pass filter time constant to be independent of the forward path gain.